1. Technical Field
The invention is concerned with a fabrication of devices best exemplified by Large Scale Integrated circuits built to submicron design rules, e.g. &lt;0.5 .mu.m. Relevant fabrication entails projection lithography using charged particle delineating energy-either electron or ion.
2. Description of the Prior Art
The relentless drive toward LSI of increasing miniaturization-of increasing chip capacity-has given rise to a number of evolving fabrication approaches. All depend on lithographic delineation of capability beyond that used in the manufacture of state-of-the-art devices--beyond that used in the manufacture of.apprxeq.0.9 .mu.m design rule, 1 megabit chips.
Presently used near-UV lithography may be extended to appreciably smaller design rules-perhaps through use of phase masks. At some level wavelength limitations for electromagnetic radiation in the near-UV, or even deep-UV spectrum, will require a different radiation source. Relevant effort at this time largely concerns use of radiation in the x-ray spectrum.
Increasingly, advantages of charged particle over x-ray delineation--of electron or ion beam delineation--is leading to investigation of this approach. Electron beam projection lithography has the potential to become a tool of choice for fabrication of &lt;0.5 .mu.m design rule devices. Experience gained, in design and construction of imaging electron optics in microscopy over a period of sixty years, and in direct-write electron beam lithography over a period of twenty years, both establishes feasibility and offers insight into appropriate apparatus/process design. Feasible accelerating voltages in the 50-200 kV range (to result in 50-200 keV electrons) translates into equivalent wavelengths of.apprxeq.0.054-0.025 .ANG.--considerably better than need for contemplated design rules--to substantially increase depth of focus, thereby relaxing criticality of processing parameters and increasing yield. Well-developed approaches to electron optics permit attainment of projection with image demagnification which facilitates fabrication of now-enlarged masks.
Use of absorbing stencil (aperture) masks imposes significant restrictions on lithography. This approach precludes annular and other such peripheral mask features--a limitation overcome by use of complementary mask pairs which, however, doubles the per-level exposure requirement with consequent increased cost in throughput, in overlay registration, and in yield. In addition, decreasing absorption accompanying increasing accelerating voltage in blocking regions of the mask forces a compromise between image contrast and resolution.
Co-pending patent application Ser. No. 390,139, filed Aug. 7, 1989 offers a process making use of implicit advantages of electron beam lithography to bypass use of aperture masks. The key feature substitutes scatter-non-scatter for absorption-transparency masking. Discrimination offered by an apertured scatter filter positioned on the ray cross-over plane before the wafer--with aperture generally on the optical axis--permits the.apprxeq.50-200 kV accelerating voltage desired for resolution and feature spacing, while offering image contrast at the 80% level and higher. The process is known as SCattering with Angular Limitation in Projection Electron-beam Lithography.
Promise for the SCALPEL approach, particularly for the fractional micron design rules contemplated, follows from reduced mask thickness as permitted by dependence on scattering angle rather than absorption. Simply stated, sufficient image contrast is achievable by means of far thinner masks. A scattering angle of the order of 50 mrad, statistically realizable by five electron-atom events (collisions or sufficiently close passage to result in meaningful deflection due to field interaction), is achievable in blocking region thicknesses of the order of 500-2000 .ANG..
Considerations including those above lead to use of a "membrane" mask. A mask, depending for feature support upon a film or membrane of a thickness of the order of 500-2000 .ANG., supporting "blocking" (scattering regions), also of a thickness of the same order, clearly yields requisite contrast for submicron design rules for at least an order of magnitude over the range below.apprxeq.0.5 .mu.m. This is appropriate for mask-to-wafer demagnification in the range of 4.times.-5.times..
As aggravated by enlarged mask size due to image demagnification, the mask, now about 5 cm wide (for 5.times.demagnification to yield a 1 cm chip), does not have the required mechanical integrity. Sagging/distortion, caused for example by local heating, resulting from partial or complete particle absorption, interferes with attainment of the extreme precision required for such design rules.